Field of the Invention
The invention relates to a method for hot forming and press hardening plate-shaped or preformed workpieces of sheet steel, in particular galvanized workpieces of sheet steel, in which the workpiece is heated to a temperature above the austenitisation temperature and is then formed and quenched in a cooled forming tool having at least one punch and at least one female mold. The invention also relates to a forming tool for hot forming and press hardening plate-shaped or preformed workpieces of sheet steel, in particular galvanized workpieces of sheet steel, having at least one punch and a female mold associated with said punch, the punch and/or the female mold having cooling ducts for conducting a cooling fluid.
Description of Related Art
Devices are known for hot forming and press hardening workpieces of sheet steel, which devices have at least two tool halves, regions of these tool halves being configured such that they have different thermal conduction characteristics which are used to enable the adjustment of locally differing strength characteristics in the component to be produced. The method implemented by these devices is known by experts as “tailored tempering”. A corresponding device is disclosed, for example, in DE 10 2009 018 798 A1.
It is also known to increase the dimensional accuracy and fitting accuracy of formed components in that the tool halves used for production have positive radii in the region of curves of the workpiece and they form air spaces in the opposite regions, projections being configured adjacently to the air spaces such that a non-warping clamping is facilitated. As a result, different hardness degrees can also be set in the component. A corresponding device for producing hardened components of sheet steel is described in DE 10 2004 038 626 B3.
Tests have shown that when galvanized steel blanks are hot formed in conventional forming tools, cracks sometimes appear in the zinc coating. The cracks can spread into the blank and thus the component may fail prematurely.
The object of the present invention is to provide a method and a forming tool of the type mentioned at the outset, which improve the flow characteristics of steel materials during the hot forming procedure, thereby significantly reducing the risk of cracks appearing during the hot forming of workpieces of sheet steel, in particular of galvanized steel blanks.